FIG. 3 is a simplified block diagram illustrating a conventional radio receiver 50 including an antennae 51 for received a radio signal RS, a receiver front end circuit 52 for converting radio signal RS into a filtered (electrical) signal VFILT, a variable gain amplifier 53 and a demodulator 54 for demodulating filtered signal VFILT to produce an output signal VOUT that is used, for example, to represent digital pulses (note that the present invention is of most benefit with digital pulse signals), or to generate music or other sounds representing information carried by radio signal RS. The specific functions performed by receiver front end circuit 52, amplifier 53, and demodulator 54 are well known in the art, and therefore a more detailed description is omitted for brevity.
It is a common practice in conventional radio receivers to compensate for the wide range of signal strengths of received radio signals RS by utilizing a comparator 55 to compare the voltage level of output signal VOUT with a reference signal VREF, and to generate a gain control signal VGC that is applied to a gain control terminal 53A of variable gain amplifier 53 in order to adjust the signal path amplification of amplifier 53 such that output signal VOUT is maintained at a near constant amplitude. Such a closed loop control system (i.e., including amplifier 53, demodulator 54, comparator 55, and filter capacitor 56) is commonly referred to as automatic gain control (AGC), and is referred to below as the AGC loop.
FIG. 4 is a waveform diagram illustrating the effect of a relatively strong radio signal RS (i.e., a radio signal having a level significantly higher than that required to start AGC action. Referring to the left side of FIG. 4, in the absence of radio signal RS, output signal section VOUT0 remains at a zero or minimum noise level. Because output signal section VOUT0 remains lower than reference signal VREF prior to time T1, comparator 55 (FIG. 3) generates a corresponding high gain control signal VGC that maximizes the gain of variable gain amplifier 53 (note that the absolute gain value depends on control phase of variable gain amplifier 53; e.g., a gain control signal VGC of 0V equals low gain, and a gain control signal VGC of 2V equals high gain). At time T1, relatively strong radio signal RS is received and causes amplifier 53 to generate an output signal VOUT1 that is substantially greater than reference signal VREF. In such a closed loop AGC system, reference signal VREF is set equal to the nominal peak output signal required, and if the initial transient is too high, this part of the signal is usually clipped and lost (this is indicated by the short horizontal section between time T1 and the downward sloped portion of signal VOUT2 in FIG. 4). In order to adjust output signal VOUT1 to the desired output level determined by reference signal VREF, comparator 55 causes gain control signal VGC to change such that the gain of amplifier 53 is reduced. This gain decrease (i.e., represented by the downward sloped portion of signal VOUT1) occurs at a predetermined rate that is determined by the response time of the AGC loop formed by amplifier 53, demodulator 54, comparator 55 and capacitor 56, whereby (as shown in FIG. 4) output signal VOUT1 gradually decreases between times T1 and T2 toward reference signal VREF.
The response time of the AGC loop is substantially determined by the output current IC of comparator 55 and the capacitance of filter capacitor 56, which produce gain control voltage VGC. That is, upon receiving a change in output voltage VOUT, comparator 55 adjusts output current IC almost instantaneously to the required output level (e.g., at time T1, current IC is reduced nearly instantaneously from a substantially maximum level to a level required to adjust output voltage VOUT1 to equal reference voltage VREF). Although comparator 55 changes current IC almost instantaneously to the required level, gain control voltage VGC, which is related to the charge stored on filter capacitor 56 in response to current IC, changes at a rate determined by the level of current IC and the capacitance of filter capacitor 56. For example, if the current IC generated by comparator 55 is relatively low and/or the capacitance of filter capacitor 56 is relatively high, then a relatively long time passes before a significant change in current IC (e.g., in response to the signal change at time T1) results in a corresponding change in gain control signal VGC.
In closed loop AGC system such as that used in radio receiver 50, it is necessary that the response time of the AGC loop is significantly longer than the slowest possible modulation rate of radio signal RS in order to avoid distortion of the signal through loss of low frequency signal components (i.e., when the signal appears to be differentiated). This issue applies to AM radio signals where peak to peak signal swing is directly affected by signal strength, and especially to AM pulse signals. Unfortunately, this long response time leads to difficulty with signals having short burst characteristics when a rapid response is required for initial settling. In particular, when radio signal RS is characterized by short bursts, this long response time produces a problem in that a large proportion of the wanted burst signal may be lost (clipped) or distorted. That is the output signal Vout, between T3 and T4, and after T5, is not at a level suitable for good reception when required. The rate of settling to the desired gain is further reduced as a result of periods T2 to T3, and T4 to T5, when the output signal is below VREF, which will cause the gain to increase. This increase in gain through AGC action when no signal is present also helps ensure that the output signal at T1 will usually be greater than previous burst signal.
Note that, although it is possible to reduce the AGC loop response time by increasing current IC and/or reducing the capacitance of filter capacitor 56, the resulting circuit would not operate in a desirable manner. As stated above, an inherent requirement of any AGC system is that the AGC loop response time must be longer than the slowest modulation rate—otherwise, if the gain control action is too fast, the control acts to distort the wanted signal. Hence, if the gain control is made faster in radio receiver 50 in order to settle transients faster, e.g., by reducing the size of filter capacitor 56, then output signal will be distorted continuously.
What is needed is a radio receiver circuit that avoids the signal “clipping” problems of conventional radio receiver circuits that are caused by a too slow response time, while avoiding the signal distortion problems associated with a too fast response time.